1. Field of the Invention
The present invention relates to a close-contact image sensing device for reading an image without an afterimage.
2. Description of the Related Art
A close-contact image sensing device using photo sensing elements, such as photo diodes, has been known as an image sensing device of the type in which an image on an original is read with the same size as that of the original image (in one-to-one corresponding manner), viz., without reducing the size of the original image.
A conventional close-contact image sensing device is shown in FIG. 5, and its basic circuit is shown in FIG. 2. In those figures, reference numeral 1 designates a photo diode as a photo sensing element; 2, an equivalent capacitor; 3, a switch for selecting the photo sensing element; 4, a capacitor existing in connection with a common signal line; 6 to 8, read lines; 9 to 11, common signal lines; 12, a drive IC; 13, a bias line; 21, a first block; 22, a second block; 2N, an Nth block; V.sub.G, a signal for selecting a photo sensing element; V.sub.L, a read signal; V.sub.P, an output signal of a photo diode.
The equivalent capacitor 2 has a capacitance that is the total of a capacitance between the electrodes of the photo diode 1, and a stray capacitance existing in connection with the photo diode 1, and others. The photo sensing element select switch 3 may be a thin-film transistor (TFT), for example. A group of the image signal read lines 6 to 8, and another group of common signal lines 9 to 11 are three-dimensionally arranged in a manner that the former group of lines intersects the latter group of lines, thereby to form a matrix circuit.
The basic circuit of the close-contact image sensing device will first be described, and then the whole of the image sensing device will be described.
The basic circuit shown in FIG. 2 will be described. The equivalent capacitance 2 is charged through a path (as indicated by a dotted line "a") originating from a bias power source V.sub.B. The same is discharged through a path of a dotted line "b" in accordance with a light incident on the photo diode 1. Accordingly, the output voltage V.sub.P of the photo diode depends on the amount of the incident light.
When the photo sensing element select signal V.sub.G is applied to the gate of the photo sensing element select switch 3, and the switch is turned on, a part of the charges stored in the lower electrode of the equivalent capacitor 2 is transferred to the common signal line capacitor 4, by way of a path indicated by a dotted line "c". A voltage across the capacitor 4 varies with the charge transfer. At the instance that the charge transfer terminates, the voltage across the capacitor is read out as a read voltage V.sub.L. As recalled, the voltage depends on the amount of the light incident on the photo diode 1. With the outputting of the voltage, an image has been read.
The closed-contact image sensing device shown in FIG. 5 will be described.
The image sensing device includes a plurality of blocks, each consisting a predetermined number of series circuits, each of which contains a photo diode 1 and a photo sensing element switch 3. In this instance, those blocks are denoted as 21 representative of a first block, 22 representative a second block, and 2N representative of an Nth block.
The select switches 3 in one block are turned on and off by a select signal V.sub.G. Accordingly, the charge transfers in one block are simultaneously performed.
The charges are transferred through the read lines 6 to 8 connecting to the select switches 3 and the common signal lines 9 to 11, which are used as transfer paths from each block.
An image on an original can be read horizontally across the original from right to left and vice versa by sequentially shifting the timing of the select signal V.sub.G applied to each block. Such outputting of the select signal V.sub.G is controlled by the drive IC 12.
In the conventional image sensing device as described above, the output voltage V.sub.P of the photo diode after the charge transfer caused in response to the turn-on of the select switch, is left. The left voltage possibly causes an afterimage.
The afterimage phenomenon will be described with reference to FIGS. 4(a)-4(c), which show a set of waveforms useful in explaining an operation of the basic circuit of FIG. 2.
FIG. 4(a) shows a waveform of the photo sensing element select signal V.sub.G ; FIG. 4(b), a waveform of the photo diode output voltage V.sub.P ; FIG. 4(c), a waveform of the read voltage V.sub.L.
Assuming that at time t.sub.1, the voltage V.sub.P, that has been increased by the charging through the path "b", is V.sub.P1, and a capacitance of the equivalent capacitor 2 is C.sub.2, a charge Q.sub.1, that has been stored, is EQU Q.sub.1 =C.sub.2 .times.V.sub.P1 ( 1)
At time t.sub.1, when the select signal V.sub.G is applied to the select switch 3, the switch is turned on and the charge transfer starts. Then, the output voltage V.sub.P of the photo diode gradually decreases, while the read voltage V.sub.L gradually increases. At the instant that both the voltages become equal to each other, the charge transfer terminates. Assuming that those voltages at the time of the termation of the charge transfer are V.sub.P2 and V.sub.L1, and a capacitance of the common signal line capaictor 4 is C.sub.4, we have EQU V.sub.P2 =V.sub.L1 =Q.sub.1 /(C.sub.2 +C.sub.4) (2).
After the voltage V.sub.L1 is fetched by the drive IC 12 (FIG. 5), it is reset at a ground level (time t.sub.3).
A charge Q.sub.2 as left in the equivalent capacitor 2 at the time of the termination of the charge transfer is expressed by EQU Q.sub.2 =C.sub.2 .times.V.sub.P2 =C.sub.2 Q.sub.1 /(C.sub.2 +C.sub.4)(3).
When light is incident on the photo diode 1, the equivalent capacitor 2 is charged again and the charging continues till the next select signal V.sub.G is inputted. The output voltage V.sub.P starts to increase from the voltage V.sub.P2. Assuming that a peak voltage that the increasing voltage V.sub.P reaches is V.sub.P3, and an amount of charge stored anew is Q.sub.3, the following relation holds EQU C.sub.2 V.sub.P3 =Q.sub.3 +C.sub.2 Q.sub.1 /(C.sub.2 +C.sub.4)(4).
At time t.sub.4, the next select signal V.sub.G is supplied and another charge transfer starts. As in the previous case, at the instant that a voltage across the equivalent capacitor 2 becomes equal to a voltage across the capacitor 4 (V.sub.P4 = V.sub.L2), the charge transfer terminates. The read voltage V.sub.L2 at this time is EQU V.sub.L2 =Q.sub.3 /(C.sub.2 +C.sub.4)+{C.sub.2 /(C.sub.2 +C.sub.4)}.times.V.sub.L1 ( 5).
In the above equation, the first term describes the charge Q.sub.3 that is charged during a period from time t.sub.2 to t.sub.4. Essentially, the charge that must be read out here is only this charge Q.sub.3. Actually, however, the component of a second term is also read out, together with the charge of the first term.
When considering the facts that V.sub.L1 =V.sub.P2 and C.sub.2 V.sub.P2 indicates the charge that is left in the equivalent capacitor 2 after the previous charge transfer, the second term indicates the component reflecting the left charge, that is, the afterimage for the precious image.
In the conventional image sensing device, C.sub.2 &lt;&lt;C.sub.4 (e.g. C.sub.2 =1 pF and C.sub.4 =100 pF) and hence a ratio of the afterimage component to the whole read voltage is small (e.g., approximately 1% if C.sub.2 :C.sub.4 =1:100). Accordingly, the afterimage is negligible.
In recent days, however, designers are pressured to remove the afterimage for the following reasons.
(1) With advance of the microfabrication technology, the matrix circuit is extremely reduced in size. As a result, the capacitance C.sub.4 of the common signal lines is small, but that ratio of the afterimage is large and is not negligible.
(2) Because of demands for further improvement of a read sensitivity, the capacitance C.sub.4 must be reduced (a denominator of the equation (2), for example, is small). This also leads to increase of the ratio of the afterimage.
(3) There is a request that an image is read at a high gradation. To this end, the afterimage must be removed.